This invention relates to metal detectors. Particularly, this invention relates to metal detectors for detecting metallic objects in a stream of predominantly non-metallic particulate matter, such as a stream of tobacco filler in a cigarette rod.
Metallic objects may be present in tobacco filler from a number of sources--for example, wire used in the baling of tobacco, hardware (bolts, hinges, etc.) detached from the hogsheads in which the tobacco is stored and transported, and shavings and filings produced by the wearing of tobacco processing machinery. It is desirable to remove such foreign objects from the tobacco before it is incorporated into cigarettes.
As whole leaf tobacco is processed into cut filler, it is subjected to screening and sifting to remove foreign objects. Large magnets are typically used to remove ferromagnetic objects. However, some metallic objects still may remain in the tobacco filler which is delivered to cigarette making machines, particularly non-ferromagnetic objects, but also ferromagnetic particles.
Known metal detection apparatus is not suitable for detecting metal particles in a cigarette rod moving at hundreds or thousands of feet per minute on a high-speed cigarette maker. Any such particles are generally too small and move too quickly for detection by conventional metal detectors. Large particles may go undetected if they are not ferromagnetic. Although some large objects may puncture the cigarette wrapper and cause the cigarette containing the object to fail a pressure drop test and be rejected, some large objects may lie along the cigarette axis without puncturing the wrapper and not cause a rejection.
It would be desirable to be able to detect small metallic particles moving at high speed in a stream of particulate matter, such as in a cigarette rod.
It would also be desirable to be able to provide a metal detector that is sensitive to very small permeability changes introduced by small metallic particles passing through the detector and is not sensitive to changes in permeability occurring outside the detector, as may exist, for example, in cigarette manufacturing apparatus.
It would also be desirable to have a metal detector that can detect both non-ferromagnetic and ferromagnetic particles.